Norovirus (NV) gastroenteritis is a major contributor to global morbidity and mortality, yet little is known about NV immunity and no effective vaccine exists. The recent discovery of murine noroviruses (MNV) a new genogroup of natural enteric mouse pathogens capable of growth in tissue culture has helped establish a key role for T cells in NV clearance. Despite these advances, a number of important questions remain, including: (1) what are the dynamics of T cell differentiation during norovirus infection~ and, (2) why do certain strains of norovirus persist in immunocompetent hosts? To address these questions we have identified novel immunodominant T cell epitopes and have constructed MHC-peptide tetramers. These new tools have allowed us, for the first time, to track MNV-specific T cell subsets and their functional properties. Based on preliminary results, we hypothesize that, in addition to being essential for initial clearance of infection, epitope- speciic T cell subsets in the intestinal mucosa can provide broad and long-lasting norovirus immunity~ conversely, loss of MNV-specific T cell function (exhaustion) in some settings leads to pathogen persistence. This hypothesis will be pursued by the following interrelated Specific Aims. (1) What are the dynamics of MNV-specific T cell differentiation, and where are T cells primed? Immunity to mucosal infection requires unique T cell differentiation states and priming mechanisms. First, using tetramer reagents, we will directly track and enumerate epitope-specific effector and memory T cells following mucosal vs. systemic infection. Second, we will use adoptive transfer to define the protective capacity of MNV-specific T cell subsets primed by mucosal vs. peripheral route. (2) Can dendritic cell (DC) vaccination induce robust T cell immunity to norovirus? Current norovirus vaccines result in weak T cell responses and this may be caused by failure of VLPs to recruit specific DC subsets necessary for mucosal T cell priming. To address this possibility, DCs from intestinal vs. peripheral sites will be exposed to immunodominant MNV T cell epitopes, and used to vaccinate nave mice. First, we will define the properties of MNV-specific T cells from DC- immunized animals before and after rechallenge. Second, we will use therapeutic DC vaccination to treat persistent MNV infection. These experiments will interface with Aims 1 and 3 by defining DC correlates of protective T cell immunity. (3) What are the differentiation states and protective properties of MNV- specific T cells that arise during persistent infection? Certain norovirus strains establish long-term infection. First, we will test whether T cell dysfunction occurs during persistent MNV infection by defining the phenotype, function, and protective capacity of MNV-specific CD4 and CD8 T cells from chronically vs. acutely infected mice. Second, we will define the reversibly of MNV-specific T cell exhaustion by using immunologic and genetic methods to selectively disrupt key inhibitory pathways. These innovative in vivo studies will provide mechanistic insights into the role of T cells in MNV infection, and serve as a platform for therapeutic strategies.